Autism is currently defined as a single disorder characterized by impairments in communication and social interaction and the presence of restricted or repetitive behaviors. The current diagnostic definition is based on behavioral characteristics, but there is mounting evidence that autism may be a collection of overlapping neurodevelopmental disorders, resulting from neuroanatomical, neurophysiological, neuroimmunologic and/or genetic abnormalities. The wide range of possible etiologies and the wide heterogeneity of symptom expression among individuals with autism has caused some to speculate that there may be several autism(s), i.e., subtypes of the disorder that have distinct causes and treatment considerations. The PDN research program aims to characterize the behavioral and biological manifestations of these subtypes of autism, in an effort to identify their etiologies and develop more effective treatments. A longitudinal phenomenological study of subtypes of autism is ongoing and we are recruiting young children with non-regressive autism and regressive autism, as well as comparison groups of children with Rett Syndrome, developmental delay, and typical development. The study includes comprehensive behavioral, neuropsychological, medical and neurological evaluations, as well as assessments of EEG activity, braing structure (using magnetic resonance imaging), sleep disturbance, environmental exposures, early medical history, genetics, genomics, and dysmorphology. Preliminary findings have revealed abnormalities of sleep architecture among children with autism, as well as epileptiform discharges observed on overnight EEG recordings. [unreadable] [unreadable] Our objective is to determine if there are distinct groups of individuals within the spectrum of autistic disorders that share common profiles of biological and/or behavioral characteristics. Identification of such subgroups will lead to hypothesis-testing research to evaluate the etiology and pathophysiology of their symptoms. We have two specific aims: 1) To characterize and validate the autism phenome; and 2) To identify neurobiologic markers for autism subgroups through the techniques of genetics, metabolomics, proteomics and genomics (e.g. gene expression profiles).[unreadable] [unreadable] Subject recruitment is ongoing for this phenotyping investigation and interested parties are invited to learn more about the study at: [unreadable] http://clinicalstudies.info.nih.gov/detail/A_2006-M-0102.html